1. Field of the Invention
The present invention concerns a cryostat for an MR magnet for medical magnetic resonance (MR) imaging.
2. Description of the Prior Art
Superconducting magnetic coils are used in medical imaging in magnetic resonance tomography (MRT) apparatuses. The superconducting magnetic coils are cooled with liquid helium. If the superconducting magnet coils are suddenly transitioned into the normally-conducting state (“quench”), the energy of the magnetic field is transduced into heat. The liquid helium is thereby vaporized and large quantities of cold helium gas must be safely conducted to the atmosphere. For this purpose, an opening is provided on the top of the tank in which the magnet is located. A structure known as the tower (or turret) extends above the opening, with a tower pipe that serves for filling the tank with liquid helium. The tower pipe transitions into the quench pipe. The diameter of the quench pipe depends on its length and its radius of curvature and is on the order to 20 to 40 cm. The diameter of the tower pipe can be smaller because the helium gas given a quench is still very cold at this point and therefore does not occupy much volume.
A particular danger is that the tower pipe of the magnet is completely sealed by air-ice droplets. Such a sealing can occur due to operating error upon refilling with liquid helium or due to leaks in the system. The seal forms from frozen air that is located in the lower, cold region of the tower. The seal withstands pressures up to several bars, such that the danger exists that the helium tank bursts. Upon a quench of an iced magnet the danger exists of the magnet exploding.
It is therefore vital for a seal in the tower pipe or quench pipe to be immediately remedied. This can ensue by radiant heat or by careful injection of warm helium gas onto the seal, but the magnet must not be caused to quench by the supplied heat. A de-energizing of the magnet is not possible if the pipe is sealed since increased helium is vaporized upon de-energizing, which would increase the pressure in the helium tank.
Detection of a seal in one of the pipes is possible in the prior art only by optical, visual monitoring, i.e. via cameras or other such sensors in the tower pipe as is described in DE 10 2005 058 650 B3, for example. A device for monitoring a tower pipe in a cryomagnet is known from this document, which has at least one monitoring unit that has a functional interaction with a state of the inside of the tower pipe of a cryomagnet to monitor the continuity of the inside of the tower pipe.
A monitoring system for a superconducting magnet as well as a corresponding monitoring method is known from U.S. Patent Application Publication No. 2006/0230769. In this system and method the quantity of liquid helium that is still present and in which the superconducting coil is located is detected. The output unit outputs the monitoring information depending on the detected remaining volume.
The solutions known in the prior art are in need of improvement.